Theoretical Study Of MOF-based CO₂ Cycloaddition Catalysts

With rapid growth of population over the world,the daily use of fossil fuels has also increased.Extensive CO₂ have been released into the atmosphere due to the combustion of fossil fuels,making the CO₂ concentration in the atmosphere rise sharply.This has triggered a series of environmental problems such as global warming,ocean acidification,climate deterioration and species extinction,seriously threatening the natural environment and human society.Thus,decreasing the atmospheric CO₂ concentration by reducing CO₂ emissions has become an urgent research topic.Scientists are dedicated in developing various strategies to reduce CO₂ emissions.Among them,producing high value-added chemical products by the cycloaddtion reaction of active substrates?such as epoxides and aziridine?and CO₂ has attracted considerable attention since the reactant atomic economy in this reaction is 100%.However,the CO₂ molecule has strong thermal stability and chemical inertness,making it necessary to introduce efficient catalyst for promoting these reactions.Metal-organic frameworks?MOFs?represents a novel type of porous crystal materials with orderly structure,ultrahigh specific surface area and good stability.These advantages equips them with high catalytic activity and selectivity for various reactions.Reacently,the CO₂ cycloaddition catalyzed by MOFs has emerged as a hot research topic.Compared with the experimental study,there are only few theoretical reports on the MOF-catalyzed CO₂cycloaddition reactions.The theoretical study can not noly allow to understand the reaction mechanism of CO₂ cycloaddition from the level of molecules/atoms,but also provide theoretical guidance for experimental research on rational design efficient catalyst.This will avoid the time-consuming trial-and-error exploration in experiment,thus accelerating the development of high-performance MOF-based catalysts for CO₂ cycloaddition.In this paper,extensive DFT calculations are performed on the MOF-catalyzed CO₂ cycloaddition reactions and the research points are focused on:1)the reaction mechanism of CO₂ cycloaddition with epoxides catalyzed by MIL-101;2)the computational screening of high-performance catalysts for the CO₂ cycloaddition with epoxides;3)the HKUST-1-catalyzed CO₂ and aziriding cycloaddition mechanism.?1?Large amounts of experimental studies have reported that MOFs can effectively catalyze the CO₂ cycloaddition with propylene oxide?PO?in the presence of ionic liquid.However,the mechanism details remain largely unclear,resulting in the difficulty for developing high-performance catalysts experimentally.Based on this,here,MIL-101 is selected as the MOFs model and tetrabutylammonium bromide?TBAB?as the ionic liquid model,the reaction mechnisms for CO₂ and PO cycloaddition in the absence of any catalyst,as well as promoted by TBAB catalyst,MIL-101 catalyst and MIL-101/TBAB catalyst are systematically explored by using density functional theory.The results show that two feasible routes?Route I and Route II?are located for the MIL-101/TBAB catalytic CO₂ cycloaddition reaction.Route I consists of two main steps:1)MIL-101 and TBAB opens synergistically the PO ring to give the intermediate Int1.2)CO₂ interacts with the intermediate Int1,thus ring-closing to generate the cyclic propylene carbonate?PC?.Route II can be divided into three steps:1)MIL-101/TBAB synergistically leads to the ring-opening of PO and give an initial intermediate Int1;2)The intermediate Int2 is generated by the interaction between CO₂ and the ring-opening intermediate Int1 through van der Waals force;3)The ring-closing step occurs in the Int2 to generate the product PC.It is found that the Route II is favorable to Route I according to the rate-determing energy barrier.Thus,the Route II is more experimentally feasible.In addition,the cycloaddition barriers for different catalysts vary greatly,presenting the ranking as non-catalysis(55.83 kcal mol^-1)>MIL-101-catalysis(45.29 kcal mol^-1)>TBAB-catalysis(27.16 kcal mol^-1)>MIL-101/TBAB-catalysis(17.15 kcal mol^-1),which is consistent with the experimental results reported previously.?2?The development of high-performance catalysts to incorporate CO₂ into cyclic carbonates has been a hot topic in the field of catalysis.It has been reported that Cu-centered HKUST-1 shows excellent performance for catalyzing the CO₂-PO cycloaddition reaction.Notably,the Cu metal centers in HKUST-1 can be replaced by a series of divalent metals with experimental techniques to obtain a series of HKUST-1 analogues with the same ligand and different metal centers?M-HKUST-1,M represents the divalent metal center?.Now,several M-HKUST-1?M=Mo,Cr,Fe,Ru,Zn,etc.?are available experimentally.According to the CO₂ cycloaddition mechanism proposed above,the catalytic active sites in MOFs are only the metal center,which belongs to single site reaction.In order to investigate the influence of metal center of HKUST-1 on the CO₂ cycloaddition reaction and to design the top-performance MOF-based catalysts,a series of M-HKUST-1?M=Mo,Cr,Ti,Cu,W,Sc,Fe,Ru,Zn,Cd,V?with different metal centers have been computationally screened for catalyzing the CO₂-PO cycloaddition by DFT calculation.The results show that all the M-HKUST-1 catalysts share the same CO₂ cycloaddition mechanism.However,their catalytic activities are significantly different.Based on the rate-determining energy barrier,the catalytic activity of several M-HKUST-1?M=Cr,W,Fe,Zn,Cd?catalysts would exceed that of the original Cu-HKUST-1.Especially,the W-HKUST-1 is predicted to be the most effective catalyst among the screening M-HKUST-1.?3?Recent experimental studies have shown that the HKUST-1 and TBAB exhibit good catalytic activity and selectivity in the CO₂ cycloaddition with aziridine under mild conditions.However,the reaction mechanism and the origin of selectivity are still unrevealed,which greatly hinders the improvement of existing MOF catalysts.Here,the reactive mechanism for the CO₂-azridine cycloaddition catalyzed by HKUST-1/TBAB is detailedly investigated with DFT calculations.The calculation results show that the cycloaddition reaction catalyzed by HKUST-1/TBAB can be divided into three mian steps,including the ring-opening of aziridine,the insertion of CO₂ and intramolecular ring-closure.On the basis of the rate-determining energy barrier,the CO₂ cycloaddition reaction catalyzed by HKUST-1/TBAB catalytic system(26.12 kcal mol^-1)is much easier than that in non-catalytic system(44.34 kcal mol^-1)and HKUST-1 catalytic system(33.62 kcal mol^-1).The preferential cleavage of the substituted C?-N over the unsubstituted C?-N bond of aziridine in the step of ring opening is the origin for the preferential formation of 5-substituted oxazolidinone as observed in experiment.In addition,for further improving the catalytic system,the catalytic effects of tetrabutyl ammonium halide?TBAX,X=F,Cl,Br,I?on the cycloaddition between CO₂ and azridine are also considered.The results show that the nucleophilicity of halide ions has significant influence on the ring-opening step and ring-closing step of the cycloaddition between CO₂ and azridine.It is predicted that the catalytic performance of TBAC is expected to exceed that of TBAB co-catalyst commonly used in experimental reported.The proton affinity energies of the halogen anion show the good correlation with the energy barriers for the ring-opening and ring-closing step,indicating that it can be used as a descriptor to screen the co-catalysts for the CO₂-azridine cycloaddition reaction.